The present invention relates to a copier capable of detecting the size of a document laid on a platen thereof. More particularly, the present invention is concerned with a copier having a platen and a platen cover whose inner surface is implemented as a mirror surface or similar reflectvie surface for, and capable of detecting the size of the document by illuminating the document and the reflective surface of the platen covering the document, reading the resulting reflections from the platen cover and document, and discriminating the platen cover and document on the basis of the read information.
It has been customary with a copier, particularly electrophotographic copier, for the operator to lay a document on a platen in a particular position matching the size of the document, cover the document by a platen cover, select one of cassettes each being loaded with sheets of particular size (e.g. A4, B3 or B5) matching a desired magnification change ratio, and then press a print button. Then, the copier starts on a sequence of copying steps. Specifically, a scanner disposed below a platen scans the document, and the resulting reflection from the document is focused into the surface of a photoconductive element to electrostatically form a latent image. The latent image is developed by a toner or similar developer stored in a developing device and then transported to an image transfer station. A sheet fed out from the cassette having been selected and waiting at a predetermining position is driven toward the image transfer station at a predetermined timing, whereby the developed image or toner image is tranferred from the photoconductive element to the sheet. The toner image on the sheet is fixed by a fixing device.
The above-described type of copier will readily reproduce even a book or similar relatively thick document if the platen cover is fully opened. This is undesirable, however, since the light directly reaching the operator's eye from the scanner is harmfful and since light entering the interior of the copier from the outside via the platen adversely obstructs accurate image reading operation. It is, therefore, necessary that the platen cover be closed even when such a thick document is reproduced.
To detect the size of a document laid on the platen, use may be made of a platen cover whose inner surface is implemented as a mirror surface or similar reflective surface. After such a platen cover has been lowered to cover the platen and a document laid thereon, the mirror surface and the document are illuminated. The resulting reflections from the mirror surface and document are incident to a CCD (Charge Coupled Device) image sensor or similar image sensor. As a result, the document portion and the cover portion are discriminated from each other, i.e., the size of the document is determined, on the basis of information having been read. While this kind of implementation is useful when the document is in the form of a sheet, it cannot accurately detect the size of a document with no regard to the thickness of the document since relatively thick documents such as books each has a different thickness.